The present invention relates to a method of removing metallic. contaminants from SIMOX substrate.
In recent years, the requirement for improvements in high speed performances of a semiconductor device has been on the increase, for which purpose has been proposed a technique for isolating a semiconductor substrate by a dielectric layer buried in the semiconductor substrate so as to reduce parasitic capacitance and wiring capacitance. The SIMOX substrate is a separation by implanted oxygen substrate wherein a silicon substrate is separated by an buried oxide layer formed by ion-implantation of oxygen. A conventional method of forming the SIMOX substrate will be described with reference to FIGS. 1A and 1B.
With reference to FIG. 1A, oxygen ions 3 are implanted into a single crystal silicon substrate 1 at a dose in the range of 0.4.times.10.sup.18 cm.sup.-2 and at a substrate temperature of around 600.degree. C. thereby forming an oxide implanted layer 4.
With reference to FIG. 1B, the substrate 1 is subjected to a heat treatment at a temperature of not less than 1300.degree. C. for a few hours to cause reaction of oxygen with silicon to form a buried silicon oxide layer 5 in the silicon substrate 1.
Metallic contaminants may be introduced into the SIMOX substrate in the oxygen ion implantation process. A result of analysis of metallic contaminants in the SIMOS substrate was reported by K. Watanabe et al. in IEEE International SOI Conference, Oct. 1994 p. 95. Through dislocation and stacking faults formed in the SIMOS substrate were reported by J. Stoemenos et al. in Proceedings of Sixth International Symposium On SOI Technology and Devices 1994, p, 16. The through dislocation and the stacking faults may cause deterioration of the device performance such as increase in leak current. There have been no reports related to removal of metallic contaminants from the SIMOX substrate.
Heavy metal contaminations to the silicon substrate do normally not exceed a detectable limitation of the already known analyzer. As a method of gettering heavy metals introduced in the device processes, inert gases such as nitrogen or argon are ion-implanted into the silicon substrate. Alternatively, a polycrystal silicon film is formed on a bottom surface of the silicon substrate which is disclosed in Japanese laid-open patent publication No. 1-235242. It is disclosed in Japanese laid-open patent publication No. 4-53140 and 4-17182 that a heat treatment and a formation of a polysilicon film are made.
As described above, the metallic contaminants, the dislocation or stacking faults may cause deterioration in resistance of the oxide film and increase in leak current. In order to obtain high performance and high reliability of the device formed on the SIMOX substrate, it is required to reduce the metallic contaminant concentration of the SIMOX substrate for suppression of the dislocation and the stacking faults. Notwithstanding, in the prior art, there have been no reports of attempts to remove the metallic contaminants from the SIMOX substrate.
If nitrogen or argon is ion-implanted into the SIMOX substrate or the polysilicon film is formed on the bottom surface of the SIMOX substrate, no substantive effect can be obtained. Namely, if nitrogen or argon is ion-implanted into the SIMOX substrate, then during a heat treatment, the heavy metal contaminants can be captured by defects formed in the ion-implantation. If the polysilicon film is formed on the bottom surface of the SIMOX substrate, then the heavy metal contaminants can be captured by crystal grains of polysilicon. Notwithstanding, if the heat treatment has been carried out to repair the crystal defects in the SIMOX substrate or to crystallization of the polysilicon toward crystal perfection, the ability of gettering the heavy metal contaminants is reduced. Normally, if the heat treatment is carried out at a high temperature of not less than 1300.degree. C., the nitrogen or argon ion implantation into the SIMOX substrate or the formation of the polysilicon film on the bottom surface of the SIMOX substrate are no longer effective.
In the above circumstances, it had been required to develop a novel method of forming an SIMOX substrate with reductions in metallic contaminant concentration and in crystal defect density.